a. Technical Field
The instant disclosure relates generally to a method and apparatus for illuminating an object, which may be an elongated object such as a pipe, bar, tube, wire, rod, or the like, for use in imaging applications.
b. Background
This background description is set forth below for the purpose of providing context only. Therefore, any aspects of this background description, to the extent that it does not otherwise qualify as prior art, is neither expressly nor impliedly admitted as prior art against the instant disclosure.
Lighting, particularly uniform lighting intensity is very important to imaging applications such as surface defect inspection. There are various types of lighting designs to accomplish such uniformity. For instance, a light homogenizer (e.g., typically a square or hexagon rod lens) can be used to distribute a non-uniform light beam into a uniform light beam over a two-dimensional (2D) area. This approach supports the uniform lighting over a flat surface.
Another approach is known as cloudy-day illumination. Automatic inspection of objects of interest (e.g., rod, bar, wire, etc.) is very common today. These objects are typically in round and long (i.e., have a circular shape in radial cross-section and are elongated). For instance, a plastic water pipe can be 10 meters long at a diameter of roughly 20 mm. An electrical wire is also subject to inspection to ensure that the insulation is properly applied. Commercially available systems for inspection of such elongated objects typically employ optical systems employing cloudy-day illumination techniques. The object to be inspected may typically be moving in its axial direction—along a longitudinal axis—while the imaging devices are arranged to image the circumference of the object as the object moves through the optical system. Multiple light sources are in use to project the light onto the object surface from all kinds of directions. This approach, however, has restrictions. First, it is limited in light intensity to that which is reflected back from the object, unless very high power light sources are used. The limited reflected light intensity results in a limited scanning rate, as well as a limited optical resolution (i.e., lower scanning rate, less resolution; or higher resolution optics typically darker). Second, the diffused lighting cannot support a fast object moving speed. Third, the light projection direction is not controlled and thus cannot be used to emphasize certain defects by selection of designed incident angles.
In addition, other approaches are known that can be used for imaging moving elongated objects, as seen by reference to U.S. Pat. No. 6,950,546, which discloses use of multiple line lights with individual light boosters, where each line light may be adjustable to form a uniform ring. Still other approaches are known, such as using directional projected spot lights along the circumference of an object.
However, objects with highly reflective outer surfaces present very challenging imaging situations. For example, processed metal objects are typically very shiny, and have mirror like reflection properties. As a result, even a segmented approach (e.g., individual line lights) may result in uniform stripes in the captured image.
Using a true optical fiber ring light, which can have a large number of fine light guides arranged over a circle, could possibly address the above-mentioned problem. However, the light, as coming out of a light guide, has a fan angle (i.e., the angle that is associated with the total reflection, or the refraction index of the light guide material; the fan angle may typically be about a 60° solid angle if the light guide is glass). Unfortunately, it is very difficult to focus the ring of light emanating from such a ring light without interruption to the uniformity.
The foregoing discussion is intended only to illustrate the present field and should not be taken as a disavowal of claim scope.